Based on current average transaction sizes, how many transactions can currently fit in a block?
If there is a sudden flood of transactions that exceed the answer to #1 how long can they remain in the mem pool to await confirmation in a later block?
There is no easy answer, since both the block size limit and the transaction sizes are variable.
currently, the block size limit sits at its low limit, which is 60 kB. A transaction size can range from a few hundred bytes to more than 60 kB. With those numbers, you could fit 1 to 100 transactions. With a typical transaction size of 2 kB, you can have 30 in a block, roughly.
Transactions will stay for a day in the pool before being dropped. This increases to seven days for those transactions were once in a block which was reorganized.
If blocks are consistently filled, the block limit will go up. Based on a spam attack in 2014, a day is enough for that limit to double. I'm not sure whether the block time change would make it slower, as I do not think the block size median window changed, but I would think it'd make the changes slower.